One method for controlling a lock-up clutch is disclosed in Japanese Patent Laid-Open No. 63-88372. According to the control method disclosed in this publication, the engaging force of the lock-up clutch is temporarily dropped to put the lock-up clutch in a slipping state in shifting a transmission when the lock-up clutch contained in a torque converter is directly coupled.
The method for controlling the lock-up clutch disclosed in the abovementioned publication allows a reduction in the shift shock occurring in shifting the transmission as compared to when the lock-up clutch is kept completely engaged. This also allows suppression of the number of times of engagement of the lock-up clutch, thus preventing busy shift, as compared to when the lock-up clutch is completely engaged after completing the shift by completely releasing the lock-up clutch until when the shift is completed in shifting the transmission.
Also, the lock-up hydraulic pressure deciding the slip amount of the lockup clutch is controlled by a control method (open loop) decided by the current shift stage, the car speed and the throttle opening angle. This can thus possibly cause abrupt changes in the engine speed (e.g., blow-up of an engine in shifting down the transmission) due to an excessive slip of the lock-up clutch or can cause a shift shock due to insufficient slip of the lock-up clutch caused by variations in the performance of the lock-up clutch itself (including an elapsed change), variations in the use conditions (e.g., viscosity of operating fluid), etc.
Japanese Patent Laid-Open No. Hei. 3-199768 also discloses technology related to a lock-up control method. This publication discloses a technique of releasing (turning off) the lock-up clutch in shifting to the lower speed stage during running while decelerating the speed. However, problems have arisen in that in a vehicle fitted with an exhaust brake, the effect of the exhaust brake drops as the lockup clutch is released, thus causing a sense of idle running. Also, the number of times of engagement and release of the lock-up clutch increases, thus increasing the sense of busy shifting and dropping the shifting feeling, by releasing the lockup clutch during shifting of the transmission.
In light of the foregoing, a need exists for a lock-up control method of an automatic transmission for making the optimum lock-up control correspond to a shift control.